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MAX3664 Datasheet(PDF) 5 Page - Maxim Integrated Products |
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MAX3664 Datasheet(HTML) 5 Page - Maxim Integrated Products |
5 / 12 page ________________Detailed Description The MAX3664 is a transimpedance amplifier designed for 622Mbps SDH/SONET applications. It comprises a transimpedance amplifier, a paraphase amplifier with emitter-follower outputs, and a DC cancellation loop. Figure 1 is a functional diagram of the MAX3664. Transimpedance Amplifier The signal current at IN flows into the summing node of a high-gain amplifier. Shunt feedback through RF con- verts this current to a voltage with a gain of 6k Ω. Diode D1 clamps the output voltage for large input currents. INREF1 is a direct connection to the emitter of the input transistor, and must be connected directly to the pho- todetector AC ground return for best performance. Paraphase Amplifier The paraphase amplifier converts single-ended inputs to differential outputs, and introduces a voltage gain of 2. This signal drives a pair of internally biased emitter follow- ers, Q2 and Q3, which form the output stage. Resistors R1 and R2 provide back-termination at the output, absorbing reflections between the MAX3664 and its load. The output emitter followers are designed to drive a 100 Ω differential load between OUT+ and OUT-. They can also drive higher output impedances, resulting in increased gain and output voltage swing. DC Cancellation Loop The DC cancellation loop removes the DC component of the input signal by using low-frequency feedback. This feature centers the signal within the MAX3664’s dynamic range, reducing pulse-width distortion on large input signals. The output of the paraphase amplifier is sensed through resistors R3 and R4 and then filtered, amplified, and fed back to the base of transistor Q4. The transistor draws the DC component of the input signal away from the transimpedance amplifier’s summing node. The COMP pin sets the DC cancellation loop’s response. Connect 400pF or more between COMP and GND for normal operation. Connect the pin directly to GND to disable the loop. The DC cancellation loop can sink up to 300µA of current at the input. When operated with CCOMP = 400pF, the loop takes approximately 20µs to stabilize. The MAX3664 minimizes pulse-width distortion for data sequences that exhibit a 50% duty cycle. A duty cycle other than 50% causes the device to generate pulse- width distortion. DC cancellation current is drawn from the input and adds noise. For low-level signals with little or no DC component, this is not a problem. Preamplifier noise will increase for signals with significant DC component. ___________Applications Information The MAX3664 is a low-noise, wide-bandwidth transim- pedance amplifier that is ideal for 622Mbps SDH/ SONET receivers. Its features allow easy design into a fiber optic module, in four simple steps. Step 1: Selecting a Preamplifier for a 622Mbps Receiver Fiber optic systems place requirements on the band- width, gain, and noise of the transimpedance preampli- fier. The MAX3664 optimizes these characteristics for SDH/SONET receiver applications that operate at 622Mbps. In general, the bandwidth of a fiber optic preamplifier should be 0.6 to 1 times the data rate. Therefore, in a 622Mbps system, the bandwidth should be between 375MHz and 622MHz. Lower bandwidth causes pat- tern-dependent jitter and a lower signal-to-noise ratio, while higher bandwidth increases thermal noise. The MAX3664 typical bandwidth is 590MHz, making it ideal for 622Mbps applications. The preamplifier’s transimpedance must be high enough to ensure that expected input signals generate output levels exceeding the sensitivity of the limiting amplifier (quantizer) in the following stage. The MAX3675 clock recovery and limiting amplifier IC has an input sensitivity of 3.6mVp-p, which means that 3.6mVp-p is the minimum signal amplitude required to produce a fully limited output. Therefore, when used with the MAX3664, which has a 6k Ω transimpedance, the minimum detectable photodetector current is 600nA. It is common to relate peak-to-peak input signals to average optical power. The relationship between opti- cal input power and output current for a photodetector is called the responsivity ( ρ), with units Amperes/Watt (A/W). The photodetector peak-to-peak current is relat- ed to the peak-to-peak optical power as follows: Ip-p = (Pp-p)( ρ) Based on the assumption that SDH/SONET signals maintain a 50% duty cycle, the following equations relate peak-to-peak optical power to average optical power and extinction ratio (Figure 2): Average Optical Power = PAVE = (P0 + P1) / 2 Extinction Ratio = re = P1 / P0 Peak-to-Peak Signal Amplitude = Pp-p = P1 - P0 Therefore, PAVE = Pp-p (1 / 2)[(re + 1) / (re - 1)] 622Mbps, Ultra-Low-Power, 3.3V Transimpedance Preamplifier for SDH/SONET _______________________________________________________________________________________ 5 |
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